Titanium is a metal with remarkable properties but its applications are restricted due to the high cost of its extraction and processing. Using the Kroll or Hunter Processes requires high purity titanium tetrachloride which is either reduced with magnesium (Kroll Process) [W. J. Kroll, Trans. Electrochem. Soc., 78 (1940) 35-57] or sodium (Hunter Process) [M. A. Hunter, J. Am. Chem. Soc., 32 (1910) 330-336]. The high purity titanium tetrachloride is produced by carbo-chlorination of the impure titanium dioxide and as all the oxides chlorinate, the impurities are removed by selective distillation of the chlorides. The other way of making high purity titanium dioxide, usually for the pigment industry, is the sulphate route where the impure titanium dioxide is dissolved in sulphuric acid and the iron, which is the major impurity, precipitated as iron oxide. However, there are several sources of titanium oxide which contain impurities or are too fine and render the conventional routes impractical. For example, titanium ores containing significant quantities of calcium oxide form in the carbo-chlorination process, calcium chloride which melts below the temperature of the fluidised bed reactor. This liquid phase de-fluidises the bed. The particle size of some other ore bodies are too fine to remain in a fluidised bed and are simply swept away. Use of the sulphuric acid route results in the formation of stable calcium sulphate when calcium oxide containing ores are leached. It would be advantageous if these materials could be simply converted into high purity titanium.
As mentioned above, there are two commercial methods, Kroll and Hunter, for the production of titanium using high purity titanium chloride with the vast majority being produced by the Kroll Process. In order to reduce the cost of titanium production, other methods have been investigated, usually starting with high purity oxide. In laboratory and pilot plant scale experiments, titanium dioxide has been reduced using calcium dissolved in calcium chloride (OS Process) [R. O. Suzuki in “Ti-2003 science and technology”. Eds G. Lutjering and J. Albrecht, (2004, Wiley-VCH, Weinheim) 245-252.] or electrochemically by electro-deoxidation in molten calcium chloride (FFC Cambridge Process) [G. Z. Chen, D. J. Fray and T. W. Farthing, Nature 407 (2000) 361-364]. In the latter process, the titanium oxide is made the cathode in a bath of calcium chloride and it is found that the cathodic reaction is not the deposition of calcium from the melt but the ionisation of the oxygen in the titanium dioxide, which diffuses to the anode and is discharged. In both these processes, ores containing calcium oxide can be treated as the calcium oxide would simply dissolve in the salt. However, there would not be any selective removal of the other elements as the final product would be a reflection of the impurities in the original feed material. Other processes, such as the Armstrong Process—‘Summary of emerging titanium cost reductions’, EHK Technologies. Report prepared for US Department of Energy and Oak Ridge National Laboratory, subcontract 4000023694 (2003) which is a derivative of the Hunter Process, all require high purity titanium tetrachloride as the feedstock.
Another process of interest, is that patented by Wainer in the 1950s [U.S. Pat. No. 2,722,509], which describes a process where equimolar amounts of finely divided chemically pure titanium carbide and finely divided chemically pure titanium monoxide were intimately admixed and heated in an argon atmosphere to form a TiC.TiO anode, a mutual solid solution of titanium carbide and titanium monoxide in which the molar ratio of the carbide to the monoxide does not exceed 1. A melt of a chloride salt of an electropositive element is used as an electrolyte and when a voltage is applied, anodic reactions of the following type occur:TiCO=Ti2++CO+xCO+2e−The titanium ions dissolve into the electrolyte, and are reduced at the cathode:Ti2++2e=Ti
Withers and co-workers have also investigated thermal and electrochemical processes for production of titanium see WO 2005/019501 and WO 2007/097823. The process involves forming a titanium oxide-carbon composite by mixing titanium oxide with a source of carbon and heating in the absence of air to a temperature sufficient to reduce the plus four valance of the titanium in the TiO2 to a lower valence and form a titanium suboxide/carbon composite electrode. In the process of forming the titanium suboxide/carbon composite electrode, any iron oxide is reduced to iron and was removed by leaching or complexing the iron in an aqueous solution at ambient temperature. WO 2005/019501 suggests that by incorporating other oxides into the anode, it is possible to reduce these other oxides at the same time, and deposit the cations simultaneously at the cathode to produce an alloy which reflects the composition of the original anode. In the same document, a method of producing high purity titanium is described which uses the same conditions as the previous experiments. These two results are totally inconsistent.
The present applicant has sought to provide a method of refining titanium from an ore comprising titanium dioxide and relatively high levels (e.g. at least 1.0 wt %) impurities including calcium oxide and iron oxide.